Bibliographic details of the publications and the web sites numerically referred to in this specification are collected at the end of the description.
The present invention is directed, in general, to liquid sampling devices and, more particularly, to the monitoring of solutes in pore water beneath surface water.
In most cases of ground water contamination discharging to surface water, the surface water body is considered to be the “point of contact” where there is a potential for contact of the contamination with flora or fauna. Because wells typically are not practical to install or sample beneath surface water, wells on land typically are considered to be the “point of compliance.” The point of compliance is the point at which the concentrations of contaminants must meet regulatory levels. However, numerous studies have shown that substantial contaminant degradation can take place in pore water in the immediate vicinity of the ground water discharge to surface water. Thus, the concentrations at the point of compliance often may substantially overestimate the actual discharge concentrations.
The ability to routinely monitor the actual discharge concentrations can potentially allow a government facility, for example, to move the point of compliance from wells to the pore water beneath the surface water body. This will take advantage of the accelerated contaminant degradation in sediment near the discharge zone and allow the facility to more rapidly achieve compliance concentrations required by regulatory agencies in ground water prior to the point of contact. If an onshore well contains contaminant concentrations in excess of regulatory action limits and a pore water sampler shows that concentrations have declined to an acceptable range prior to discharge to surface water, substantial cost savings can be achieved by the closing of the monitoring site.
There is strong interest on the part of the U.S. Department of Defense (DoD) to develop the ability to monitor contaminant discharge to surface water. For example, programs are underway to design an extensive environmental monitoring system at Marine Corps Base Camp Lejeune and surrounding environs in east central North Carolina as part of an effort to address military and other impacts on the New River Estuary in North Carolina. In addition, the DoD has requested the development of technology that will allow the long-term monitoring of chlorobenzene contamination in pore water beneath surface water at the Naval Air Station in Corpus Christi, Tex. Existing technology is inadequate to perform this task.
Existing technology for sampling contaminants in pore water beneath surface water includes diffusion samplers. Diffusion samplers are polyethylene bags or other material filled with water or sorbent that are used in wells or other environments to passively sample volatile organic compounds (VOCs) (see Vroblesky and Hyde [8], Karp [4], and Imbrigiotta et al. [2]). Diffusion samplers include passive diffusion bag (PDB) samplers that have been widely used. A user's guide for PDB samplers has been published (see Vroblesky [10] and [11]). Passive vapor samplers have been developed for sampling beneath surface water (see Church et al. [1] and Vroblesky [9], [12], and [13]). Diffusion samplers for inorganic constituents have also been used (see Vroblesky et al. [14] and [15]). Nylon-screen diffusion samplers have been used to sample interstitial pore waters in wetlands sediment (see Paludan and Morris [7]).
Most diffusion samplers, including the ones mentioned above, are not rechargeable. In other words, the non-rechargeable diffusion samplers are one-time use instruments that require removal for recovery and replacement of a new sampler. The specific sampling point cannot be resampled for confirmation. Thus, non-rechargeable diffusion samplers have limited value for long-term monitoring of a specific sampling point.
A rechargeable dialysis pore water sampler has been tested (see Jacobs [3]), but this type of sampler must be refilled with deionized, deoxygenated water. Creating deoxygenated water can be time consuming and troublesome. A pore water sampler is needed that does not require refilling with any kind of water, and that allows samples to be collected from the same location indefinitely, assuming sufficient time elapses between sampling events for the ambient water to return to pre-pumped conditions.
A broad variety of other diffusion-type samplers are available and have been reviewed in a recent journal article (see Namienśnik et al. [5]).
Wells sometimes are installed in surface water bodies to monitor pore water concentrations, but wells are limited in their application. Wells require a surface expression, such as a standpipe, that may be subject to disturbance by floating objects or currents. Moreover, bottom sediment is often fine grained, necessitating the need for a sand pack or other sediment-filtering material surrounding the well screen.
Well screens with prepacked sand packs are widely available. Examples can be found at the websites of the following companies: Geoprobe Systems [16], Johnson Screens (India) Ltd. [17], Weatherford International Ltd. [18], and Baker Hughes Inc. [19]. These well screens are designed to be attached to well pipe that extends below land surface. The designs are available as wire-wrapped and slotted pipe and typically have a sand pack attached to them that is held in place by a stainless steel screen. The existing prepacked well screens are designed to be attached to well casing and, thus, are not applicable to monitoring pore water beneath surface water bodies.
Some prepacked screens are designed for oilfield use and are very complex. For example, see the prepacked system shown on page 5 of the Schlumberger Limited website (see Schlumberger Limited [20]).
Some prepacked well screens are expensive because stainless steel pipes are used. An example can be seen at the website of Johnson Screens (India) Ltd. [17]. The strength and durability provided by stainless steel pipes are not needed for the less pressurized environment of the bottom-sediment pore water. Although some prepacked well screens have a small-diameter pipe and are constructed of mostly plastic material, such as the screens available from Geoprobe Systems (see Geoprobe Systems [16]), the prepacked sand in these wells is held in place by a stainless steel well screen. The stainless steel is needed because a less expensive material, such as nylon mesh, is not durable enough to withstand deployment abrasion. However, some of the parameters of interest in pore water investigations may be adversely affected by the stainless steel. Stainless steel in contact with water can generate hydrogen (see Bjerg et al. [6]). Because hydrogen is of great interest in microbial investigations of ground water, the potential for stainless steel to produce artificially high hydrogen concentrations makes stainless steel an undesirable option for this type of sampling. In addition, stainless steel is not recommended for metals sampling (see Ministry of Environment [21].) A pore water sampler is needed that is not constructed of stainless steel.
Some existing prepacked screens are designed to sample aquifer sediment, which typically consists of sands or gravels. However, bottom sediment beneath surface water bodies usually is substantially different than aquifer sediment. Bottom sediment often is fine grained and in quiescent water can consist of a substantial amount of forest detritus. This detritus can consist of partially decomposed leaves, which have a sheet-like character. A single leaf, when pulled against a screen or a smooth, slotted surface (such as the surface of the lysimeter described in U.S. Pat. No. 4,923,333), can wrap around and block a large amount of the screen opening, cutting off flow. A pore water sampler is needed that is able to counteract the potential for leaf debris to blanket and obstruct large parts of the screen.
Typical prepacked screens used in oil technology are longer than 10 feet. Small-diameter prepacked screens for ground water monitoring are available in 3-feet and 5-feet lengths. However, substantial changes in water chemistry can take place in bottom-sediment pore water over distances much less than 3 feet. Therefore, a smaller sampler is needed for this environment.
Thus, a pore water sampler is needed that is small in size (less than 3 feet), is constructed mainly of plastic or polymers, does not need to be refilled, and can counter the potential blockage effects of sheet debris.